This application claims priority under 35 USC xc2xa7119 to Finnish Patent Application No. 20011828 filed on Sep. 17, 2001.
The present invention relates to a method for performing positioning, comprising the steps of receiving a signal transmitted by satellites that is spread spectrum modulated with a repetition code, performing acquisition to the received spread spectrum modulated signal, measuring the code phase of the received spread spectrum modulated signal, receiving satellite ephemeris parameters which are used in the positioning, determining a cost function, and performing an optimizing phase for minimizing the cost function. The invention relates also to an electronic device comprising means for performing positioning, comprising means for receiving a signal transmitted by satellites that is spread spectrum modulated with a repetition code, means for acquisition to the received spread spectrum modulated signal, means for measuring the code phase of the received spread spectrum modulated signal, means for receiving satellite ephemeris parameters to be used in the positioning, means for determining the cost function, and means for minimizing the cost function.
In positioning systems based on satellite positioning, a positioning receiver attempts to receive signals of at least four satellites in order to detect the position and the time data of the positioning receiver. An example of such a satellite positioning system is the GPS system (Global Positioning System), comprising a plurality of satellites orbiting the globe according to predefined orbits. These satellites transmit orbit data, on the basis of which the position of a satellite can be determined at each moment of time, in case the exact time data used in the satellite positioning system is known in the positioning receiver. In the GPS system, the satellites transmit a spread spectrum signal modulated with a code that is individual for each satellite. Thus, the positioning receiver can distinguish signals transmitted by different satellites from each other by using a reference code corresponding to a satellite code generated locally in the positioning receiver.
Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, these signals are modulated with at least one pseudo sequence. This pseudo sequence is different for each satellite. As a result of the modulation, a code-modulated wide-band signal is generated. The modulation technique used in the receiver makes it possible to distinguish between the signals transmitted by different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, the pseudo sequence used for modulating the L1 signal is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with the polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to generate different C/A codes by using identical code generators. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the xe2x80x9chealthxe2x80x9d, orbit, time data of the satellite, etc.
In order to detect satellite signals and identify satellites the receiver must perform acquisition, whereby the receiver searches for the signal of each satellite at the time and attempts to perform acquisition and to be locked to this signal so that the information transmitted with the signal can be received and demodulated.
The positioning arrangement has two primary functions:
1. to calculate the pseudo range between the receiver and different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudo ranges and the position data of the satellites. The position data of the satellites at each time can be calculated on the basis of the ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudo ranges, because the time is not accurately known in the receiver. Thus, the determinations of position and time are iterated until a sufficient accuracy is achieved with respect to time and position. Because the time is not known with absolute precision, the position and the time must be determined e.g. by linearizing a set of equations for each new iteration.
The calculation of pseudo range can be performed, for example, by measuring the code phases of the satellite signals in the receiver.
The above-mentioned acquisition and frequency control process must be performed for each satellite signal received in the receiver. Some receivers may comprise multiple receiving channels, wherein an attempt is made in each receiving channel to perform acquisition to a signal of one satellite at a time.
The positioning receiver receives information transmitted by satellites and performs positioning on the basis of the received information. In order to perform positioning the receiver must receive a signal transmitted by at least four different satellites to detect the x, y, z coordinates and the time data. The received navigation information is stored in a memory, wherein this stored information can be used for detecting e.g. the ephemeris data of satellites.
FIG. 1 shows, in a principle chart, positioning in a wireless communication device MS comprising a positioning receiver by means of a signal transmitted from four satellites SV1, SV2, SV3, SV4. In the GPS system, the satellites transmit ephemeris data as well as time data, on the basis of which the positioning receiver can perform calculations to determine the position of the satellite at a time. These ephemeris data and time data are transmitted in frames (not shown in the appended figures) which are further divided into subframes. In the GPS system, each frame comprises 1500 bits which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe will thus take 6 s, and the whole frame will be transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each subframe 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information on the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for transmission of ephemeris data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data of all the satellites. The entity of these subframes and frames is called a GPS navigation message, which comprises 25 frames, i.e. 125 subframes. The length of the navigation message is thus 12 min 30 s.
In the GPS system, time is measured in seconds from the beginning of a week. In the GPS system, a week begins at midnight between Saturday and Sunday. Each subframe to be transmitted contains information on the moment of the GPS week when the subframe in question has been transmitted. Thus, the time data indicates the time of transmission of a certain bit, in the GPS system this is the transmission time of the last bit in the subframe in question. In the satellites, time is measured with high-precision atomic chronometers. In spite of this, the operation of each satellite is controlled in a control centre for the GPS system (not shown), and e.g. time comparison is performed to detect chronometric errors in the satellites and to transmit this information to the satellite.
It is an aim of the present invention to provide a new method for carrying out positioning. The invention is based on the idea that a new cost function is formed, and the minimum of this cost function is searched for. To be more exact, the method according to the present invention is mainly characterized in that the cost function includes a fraction operator having an argument that includes at least one residual error.
The electronic device according to the present invention is mainly characterized in that the cost function includes a fraction operator having an argument that includes at least one residual error.
Considerable advantages are achieved by the present invention when compared with positioning systems and receivers of prior art. Upon applying the method according to the invention there is no need to indicate the navigation data sent together with the satellites. Thus, positioning can be performed also from a noisier signal, which improves the usability of positioning even in weak signal conditions, such as indoors. Because it is not necessary to receive the frames used in the transmission of navigation data before positioning, applying the method of the invention enables faster positioning and shorter detection time for Time To First Fix (TTFF) compared to prior art receivers. Furthermore, in the method according to a preferred embodiment of the invention it is not necessary to guess the position and the reference time of the receiver in advance, instead this information can be found out during a calculation carried out in the positioning. Moreover, in this embodiment the error of the reference time of the receiver can be as much as over 420 s, which in prior art receivers would mean that the correct positioning of the receiver could not be carried out.